Traditional miniature wire-wound magnetic circuit components are relatively simple in design compared to the miniature multilayer magnetic components manufactured using the materials system of this invention. Unfortunately this simple design of the traditional miniature wire-wound magnetic components is not conducive towards current trend of increased functionality and integration in electronic design and manufacturing of the circuit boards so as to reduce cost, size and, weight of the device. In the traditional miniature wire-wound magnetic component a very tiny wire is wound around a magnetic core to form the induction coil. Automation of the coil winding process is not feasible for such very small sized components. Hence manufacturing of such very small sized components is costly due to the use of a labor-intensive manual wire-winding process. By design, these miniature magnetic circuit components have following two shortcomings: (1) too large a form factor (height in relation to base of a component mounted on the circuit board) compared to that of the chip scale components needed for size and weight reduction of electronic devices of today and tomorrow and (2) cannot be integrated into the typical multilayer microelectronic manufacturing process to design a multi-functional/increased functionality component with inductive as well as capacitive and/or resistive functions for potential lowering of cost, size and weight of the electronic device. This approach to potential cost reduction by increasing the functionality of a device is well known to those knowledgeable in the art of hybrid microelectronics and semiconductor devices and packages. Part of this trend is being accomplished by an ongoing increased level of integration on the IC (Integrated Circuit) chips by the semiconductor manufacturers. Even so a large number of passive components are needed to interconnect and support these ICs. Thus, there is an ongoing effort to miniaturize these passive components so as to reduce the size and weight of the circuit board. Miniaturization of resistors and capacitors has been accomplished by utilizing techniques such as discrete surface mounted chip components and thick film buried components. Miniaturization of the conventional wire-wound magnetic components such as inductors and transformers has been conceptually demonstrated by replacing the wire-wound induction coil with a multilayer microelectronic design that uses a thick film conductor deposited on laminated layers of a ferromagnetic ceramic to form the induction coil of the magnetic component. These multilayer magnetic components are manufactured in accordance with processing steps typical of LTCC and thick film technologies. Compared to the conventional wire-wound miniature magnetic component the multilayer component design has following advantages: (1) magnetic component with a chip scale form factor for potential reduction in circuit size and weight, (2) potential to lower manufacturing cost by process automation and, (3) potential for additional lowering of device cost with increased functionality by integrating inductive as well as capacitive and/or resistive functions on a component.
U.S. Pat. Nos. 5,312,674; 5,349,743 and 6,054,914 disclose designs and methods for manufacturing non wire-wound, monolithic, multilayer transformers (magnetic circuit components) using Low Temperature Cofired Ceramic (LTCC) technology and High Temperature Cofired Ceramic Technology. A thick film paste or ceramic green tape of a ferromagnetic material is used to form a single magnetic layer. Each layer acts as a substrate for the next layer in the sequential build-up of the multilayer magnetic component. Pluralities of such magnetic layers are laminated on top of each other to form the multilayer transformer. When using thick film paste of the ferromagnetic material, the multilayer magnetic component is manufactured by laminating individual layers of a dielectric ceramic green tape screen-printed with the ferromagnetic paste. Thick film conductors are screen-printed on individual magnetic layers to form a part of the electrical winding of the transformer. The primary and the secondary windings can be placed on the same magnetic layer or spread vertically over several magnetic layers through the multilayer component. When such windings extend over more than one magnetic layer vias or holes are provided at appropriate locations through the magnetic layers to facilitate interconnection between the windings on different magnetic layers. These vias are filled with the thick film conductor to complete the electrical interconnection. In accordance with the typical multilayer component design and manufacturing procedure each layer is independently punched with v as and screen-printed with appropriate thick film pastes as needed. Then all of these layers are laminated in appropriate sequence to form a green multilayer component package that is fired into an integral structure at an appropriate high temperature.
U.S. Pat. No. 6,198,374 discloses use of a lower permeability dielectric on Nickel-Zinc-ferrite (Ni—Zn ferrite) layers to improve the magnetic coupling coefficient of a multilayer transformer and to improve the dielectric breakdown voltage between the adjacent conductor layers in the multilayer transformer. Without the use of the lower permeability dielectric, the transformer design shown in U.S. Pat. No. 6,198,374 has uniform magnetic permeability throughout the multilayer structure with calculated theoretical magnetic coupling coefficient of 0.83 and a breakdown voltage of 2400VAC with a 7-mil thick ferrite layer (345VAC/mil for a 7-mil thick ferrite layer). By applying the low permeability dielectric over the thick film conductor in specific areas the magnetic coupling coefficient of such a transformer is improved to approximately 0.95 with improved dielectric breakdown voltage.
The concept of non wire-wound magnetic components dates back over a decade as shown in U.S. Pat. Nos. such as U.S. Pat. Nos. 3,833,872 and 4,547,961. During these past few years have appeared miniature multilayer non wire-wound transformers. These multilayer transformers of earlier inventions used materials systems that were not necessarily optimized for such applications. As a result multilayer transformers manufactured with these existing materials systems had shortcomings in at least one of the following typical application requirements for a miniature magnetic circuit component: (1) Magnetic coupling coefficient less than 0.95, (2) Dielectric breakdown voltage less than 1500V, a typical value for a miniature wire-wound transformer (3) Form factor larger than chip-scale, i.e. thickness of the transformer in relationship to its area for mounting on a circuit board (4) Commercially proven multilayer magnetic component design and, (5) Higher or comparable cost compared to typical miniature wire-wound transformers. These shortcomings have contributed to the slow pace of commercialization of this multilayer technique towards miniaturization and replacement of conventional wire-wound magnetic components such as inductors and transformers.
Tape casting, also known as doctor blade casting or knife casting is a well known technique used for casting thin, flat, sheets of a ceramic material using a slip of the said material. The tape casting slip consists of organic and inorganic components. In accordance with their function in a tape casting slip these components can be classified as follows: (1) The primary ceramic powder, (2) Fluxes and sintering aids to assist in densification of the fired ceramic, (3) Fillers or additives to adjust application specific properties of the fired ceramic, (4) Resin to bond all the inorganic particulates together in the green or dry, unfired state, (to form a green tape), (5) Plasticizers to modify the properties of the resin so as to make the ceramic green tape flexible for forming or shaping and laminating, (6) Solvents as a medium to dissolve the resins and plasticizers and suspend the inorganic particulates to form the slip and, (7) Surfactants to facilitate homogeneous dispersion of the particulates in the slip. In a basic tape casting process a doctor blade is used to uniformly spread the slip over a moving carrier film made of materials such as silicone coated polyester. A ceramic green tape is obtained by evaporating the solvent from the wet film. The left over resin and plasticizer hold the ceramic particulates together while providing sufficient flexibility to the ceramic green tape for subsequent forming operations such as cutting to desired size and shape, drilling holes and, lamination of multiple layers of the ceramic green tape to form a multilayer ceramic green tape package (green package). After all of the forming operations are complete the green package is processed at an appropriate high temperature using a material specific time-temperature firing protocol (firing profile) to burn-off the remaining organic components and sinter and densify the remaining inorganic components to form the fired multilayer ceramic package (fired package). During this entire process, beginning with making of the slip to cooling of the fired package to ambient, all materials and processing related variable factors can potentially influence the properties of the resultant fired package. With reference to one specific application Berry et al. (“A Design of Experiment for a Tape Casting Process”, C. Berry et. al, Proc. 2000 International Symposium on Microelectronics, pp.150-155) identified over forty possible variable factors in the tape casting process and by utilizing the design of experiments technique concluded that only six of these factors were critical for manufacturing LTCC tapes with acceptable consistency and repeatability for their application.
A typical thick film ink or paste for screen-printing contains, at minimum, particulates of an inorganic active ingredient suspended in an organic screening agent also known as vehicle. A typical screening agent is a solution of a high molecular weight polymer dissolved in a high boiling point alcohol. The ink is processed in accordance with the typical print and fire thick film technique to obtain a fired film of the active inorganic ingredient on a substrate. The sintered active ingredient dominates the electrical properties of the fired film. In practice the performance of the fired film is also affected by materials related variables such as shape and size of the particulates, chemical reactivity and concentration of the solid phases in the film. Usually thick film pastes are classified in accordance with their intended application and as such the dominant active ingredient has electrical properties typical of that general class of materials.
Following flow chart illustrates the typical procedural steps generally carried out in LTCC processing of a microelectronic multilayer magnetic component.
Typical LTCC process flow chartCast LTCC ferrite tape↓Cut tape to size↓Punch registration and via holes↓Fill vias with conductor paste and dry↓Screen-print and dry buried conductors↓Screen-print and dry dielectric layers↓Screen-print and dry top/solderable conductor↓Laminate↓Cut to size↓Fire
When using a thick film ferrite paste to fabricate the multilayer magnetic component the same general procedure is followed except a substrate is used as a base for the multilayer component and the thick film ferrite and conductors are deposited layer-by-layer per the part design. The layers may be separately fired or cofired. In the separate firing process each layer is deposited and fired and forms the basis for the next layer in the part build up.